Right. Create your slideshow in iPhoto and export it as a quicktime movie to your desktop using the "Large" setting. Then drag that quicktime movie into your iDVD project as a movie, not a slideshow. (In iDVD, go to the Project menu and click on Add Movie, and then drag your movie into the "Add Movie" button that appears in the iDVD menu window.) If your movie is longer than 1 hour, burn it at Best Quality, if under an hour, use Best Performance.
Incidentally, at the bottom of the iPhoto slideshow window there is a box called "Adjust" that will let you adjust the display time for individual slides, as well as vary the transition speed. There's also a box that lets you add/adjust the Ken Burns effect to a limited extent.
As mentioned by Kasa, there are other ways to make slideshows, but with as many photos as you have, I think that this would be the easiest way amongst the ways listed by Kasa. If you used iMovie to create your slideshow, you would have more options, such as titles and varying transitions and effects, but then its more work. Tradeoffs, always tradeoffs. 🙂